Ribulose-1,5-bisphosphate carboxylase oxygenase catalyses the important reactions of photosynthesis and photorespiration. While we have gained considerable knowledge of the catalytic function of the large subunits in recent years, very litle information concerning the functional role of the small subunit is available. Although a regulatory role has been assigned to the small subunit but there is no direct evidence to substantiate this. Since no one has been able to successfully dissociate and reconstitute this enzyme in vitro, why not let the plant perform this trick with its own devices. This is the heart of this proposal. By taking advantage of the variability and mode of inheritance of the subunits of this enzyme within Nicotiana, many desired combinations of large and small subunits can be constructed in vivo. Enzymes composed of identical large and different small subunit can be constructed by selected interspecific crosses and backcrosses. This is certainly an excellent alternative approach to the biochemical reconstitution, since it causes no damage or modification to the molecule, no change or loss of activity and, above all, introduces no artifacts because plants are doing all the engineering work. The specific activity and kinetics of activation and catalysis of this new enzyme can be found rigorously studied and tied to its structure. Difference of its enzymatic activity found in the reconstituted enzyme would be attributed to alterations of the small subunit. Therefore, results derived from this proposal should allow direct demonstration of a defined functional role of the small subunit.